 My friends, here you are, hello. So today, we are going to finish our conversation about the immune system. In the last lecture, we looked at the innate immune response. It's present from birth. It's super fast. It's completely nonspecific. Same reaction for anybody who invades. And this time, we're going to look at the acquired immune response. The acquired immune response is very specific, like shockingly, crazily specific. There's a whole bunch of molecules that are involved in making sure, are we really serious about activating this specific immune response because it's like super hardcore. So it's facilitated by lymphocytes. We're going to talk about what these lymphocytes are. But before we do that, let's just look at an overview, like a general, this is what happens in the acquired immune response. First of all, the players, the lymphocytes, pated by lymphocytes, in two flavors, B lymphocytes and T lymphocytes. So these are the cells that do the job. And I'm not going to draw the specific ones yet, but they have very specific receptors. And each lymphocyte has a very specific receptor. So the first thing, the first requirement, is that every single one has a unique receptor to some possible antigen that may or may not ever invade your body. Once the receptor, this is my little receptor, and let's make sure we're all clear that it is a very specific receptor, the receptor, whatever, whether it's a B cell receptor or a T cell receptor, it's going to combine, if it combines with its specific antigen. So you can imagine that if there was a very specific antigen that just matched this receptor, that's the next step. And then it's not even, like, it's so powerful that there's a whole layer of, like, checks and balances. Like, that's awesome. You found your antigen, your very specific antigen. You found it, but now you're going to have to go through this check point to find out. And I'm going to tell you right now, this is like, you have to be activated. Even if you find your specific antigen, the lymphocyte finds the antigen, it has to be activated. And it has to be activated by someone who has an MHC2 platform. Now, I told you about MHC in the previous section, in the previous lecture, so you kind of have an idea. I'm going to spend two whole portions of this lecture talking about MHC platforms. But the lymphocyte has to find someone with an MHC2 platform to be activated. If it gets activated, if we have a successful activation, that's when it's not even, it's, like, scary huge, how big the response, which is basically, fight. Did you follow that? Bind with a specific receptor. You find your antigen, you have to be activated by someone with an MHC2, and then you can go to battle. Now, there's two different, there are two different, I don't know, layers of specifics, depending on whether you're a T cell or a B cell. So we're going to look at that. The T cells are involved in cell mediated immunity, and the B cells are involved in humoral immunity. Those are two different pathways. So we're going to have different specifics that we're going to fill in here, but the pattern is the same between them. Notice this is really pretty different than the innate immune response. Let's start out by looking at our friends, the BNT lymphocytes, and seeing what's the scoop with these guys, what is the story, who are they, what are their unique characteristics. Let's be friends.